It should be understood that the term "SOI" herein used is meant to broadly include the structure having a semiconductor crystal layer on an insulating film and is not limited to the structure having a silicon crystal layer on an insulating film. There is known a method for producing a SOI substrate, wherein an opening is formed in an insulating film on a semiconductor substrate and a semiconductor crystal layer is epitaxially grown on the insulating film using as a seed a semiconductor crystal of the exposed semiconductor substrate.
FIG. 13 shows a sectional view of a SOI substrate epitaxially grown according to the conventional method. In the method, an insulating film 22 is formed on a semiconductor substrate 21, followed by removing a portion of the insulating film 22 to form an opening 23. Then, using as a seed a semiconductor crystal exposed in the opening 23, an epitaxial layer of the same kind as or different kind from the semiconductor crystal is grown to form a semiconductor crystal layer 24 within the opening 23 first. The epitaxial growth being further continued, the semiconductor crystal layer 24 is laterally grown on the insulating film 22.
In the conventional method for obtaining a SOI substrate utilizing lateral epitaxial growth, however, the semiconductor crystal layer is epitaxially grown vertically as well as laterally on the insulating film. As a result, the semiconductor crystal layer grows thicker on the opening than on the insulating film as shown in FIG. 13, making the thickness of the crystal layer non-uniform. Accordingly, the conventional method cannot offer a semiconductor crystal layer having a uniform thickness and covering a large area.
In addition there is another problem that since each element is isolated from another by means of pn junction in a conventional semiconductor device, parasitic capacitance is generated to prevent a high-speed operation of the device or lead to latch up.